The development of the integrated circuit ("IC") has led to the establishment of a multi-billion dollar industry. The ability to miniaturize electrical circuitry has found countless applications. Equipment that was formerly large and cumbersome can now be reduced greatly in size, while retaining and/or expanding the equipment capabilities. The development of the hand-held calculator is one of the more dramatic examples of an IC application.
With the advancement of IC technology, many of the products themselves have become inexpensive commodities. This statement refers specifically to the chips themselves, which are sold in a very competitive commodity market. This situation tends to create a shift of emphasis from the product to the manufacturing process. When a manufacturer does not have a significant technological advantage, he must either make his product with higher quality or sell it for a lower price in order to remain competitive. Therefore, the manufacturing process comes under more exacting scrutiny.
One element of the IC manufacturing process is the interconnect bonding which occurs during construction of the chip. There are at least four techniques in the prior art for accomplishing interconnect bonding.
A first technique is wire bonding. Wire bonding is accomplished either ultrasonically, by utilizing high frequency pressure waves to rapidly rub the subject leads together until a bond is formed; thermally, by forming a weld through heating under pressure; or a combination of both thermal and ultrasonic bonding. A disadvantage of this technique is that it is relatively slow due to the fact that each wire end must be individually connected. A further disadvantage of wire bonding is that IC technology has reached the density limits imposed by wire bonding.
A second technique is tape automated bonding ("TAB"). TAB bonding makes use of a photolithographic metallized plastic film. TAB is utilized in the same way as wire bonding, but for mass group applications. The tape material may be applied by soldering, or by ultrasonic or thermal bonding. A disadvantage of TAB is that expensive tooling is required for mass bonding, which is a widely required process. Another disadvantage of TAB is that due to the geometry of the method, a considerable area on the dies' perimeter is occupied, thus limiting the packing density of dies. Finally although TAB allows greater connection density than wire bonding, IC technology has also reached the limits imposed by TAB.
A third technique is to simply use a solder bump. A bump of lead or gold tin solder is applied to the connection site and then heated so that it liquefies. The bump then solidifies as it cools so that a conductive joint is formed. A disadvantage of the solder bump method is that it requires the presence of metal above active circuitry, thus slowing electrical signal propagation. Further, the solder bump requires relatively large attachment sites and cannot be inspected after reflow soldering. Density limitations limit the usefulness of this technique as well.
The fourth technique is a form of cold interconnect bonding. This method entails the use of gold/indium bumps at the connection site. This method is based upon the fact that indium plasticizes at about 29.degree. C. (85.degree. F.). Therefore when moderate pressure, without added heat, is applied, the indium will liquefy. When the pressure is removed, the indium will solidify so that a bond is formed at the connection site. A disadvantage of the indium bump bond is that it has limited usage since, due to the fact that indium plasticizes at around 20.degree. C. (85.degree. F.), it can only be utilized in a system that does not experience elevated temperatures due to heat accumulation, e.g. a cryogenically cooled system. Moreover, indium bump bonds are extremely fragile and are unusually sensitive to contamination during fabrication and while in use. In sum, all the bonding methods in the prior art, due to geometric and other considerations, limit the extent to which IC technology can advance. Further the processes required to achieve the bonds may be relatively lengthy.